[Android Webview] Disable pictureInPictureEnabled.

This makes sure Picture-in-Picture is disabled on Android Webview as
there is no UI implemented.

Bug: 852449
Change-Id: I526e02b0c2210fb5d71883f9db4ab05d76c56d2c
Reviewed-on: https://chromium-review.googlesource.com/1100464Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: François Beaufort <beaufort.francois@gmail.com>
Cr-Commit-Position: refs/heads/master@{#567341}

android_webview/browser/aw_settings.cc

@@ -54,6 +54,7 @@ void PopulateFixedWebPreferences(WebPreferences* web_prefs) {
   web_prefs->shrinks_standalone_images_to_fit = false;
   web_prefs->should_clear_document_background = false;
   web_prefs->viewport_meta_enabled = true;
+  web_prefs->picture_in_picture_enabled = false;
 }
 
 const void* const kAwSettingsUserDataKey = &kAwSettingsUserDataKey;